Running speakers active in a nutshell, is running each driver on its own channel from an amplifier, receiving a specific bandwidth; tweeters receiving the high frequencies, woofers receiving the mid frequencies and subwoofers receiving the low frequencies. The difference between an active and passive setup is how the speakers are crossed over. A passive setup passes the power from the amplifier through a series of inductors, capacitors and resistors in order to pass the correct frequencies to the drivers. On the other hand, in an active setup, the frequencies are filtered electronically, via the head unit or an external electronic crossover. That signal is passed to the amplifier, where each speaker gets its own channel.

http://i2.photobucket.com/albums/y13/goatboy_521/active.jpg

The tweeters have a high-pass crossover, and usually receive frequencies above 2 to 4 Kilohertz (2,000hz to 4,000hz and above.) The woofer or midrange is run on a bandpass filter, which consists of a combination of a high-pass and a low-pass crossover. They handle the frequencies between roughly 60 to 80 hertz and 2 to 4kHz. The subwoofer is run on a low-pass crossover, which means it plays frequencies below 60 to 80 hertz.

Also important to an active setup is the crossover slope. There are several different sloped, with the most typical being -6db/oct, -12db/oct and -18db/oct. While that may seem like Greek, it’s simple- the higher the number, the steeper the slope. A shallow slope allows a more gradual rolloff above or below the crossover frequency, while a steeper slope mandates a more immediate rolloff. Crossover slopes can influence the sound in that it affects how the drivers blend together, as well as affecting the mechanical limits of a driver. For example, if you have your mid crossed over at 80hz with a -6db/oct crossover slope, it will produce more bass than it would’ve with a -12db/oct because it is receiving more bass frequencies, but with the price of lowered mechanical power handling, and possibly damaging the suspension if the speaker is pushed beyond its limits.

An advantage of having an active setup is the ability to independently attenuate each driver. If one of the drivers is overpowering the others, you can simply go into your head unit’s menu or access the gain controls on the electronic crossover, and attenuate whatever driver is overpowering the others in order to maintain proper staging. Also when using the active crossover in a head unit, you can independently adjust the time alignment and phase of each individual driver for precise imaging.

The biggest disadvantage to running active is that it requires lots of amplification. Instead of just having a 2-channel for your front speakers and a mono amp for your subs, it requires a 4-channel for your mids and highs (or even a 4-channel and a 2-channel if you’re running a 3-way active front stage consisting of midbass, mid and tweeter) Also, it requires a lot more wiring and tuning, as it is much more complex than just running passive crossovers.